The recognition structures of both B cells and T cells have been examined using anti-receptor antibodies prepared against monoclonal anti-H-2 antibodies (mAb) and cytotoxic T cell (CTL) clones. Anti-idiotypic antibodies (anti-Id) were prepared against several monoclonal anti-H-2 antibodies. In one case, anti-idiotypic antibodies prepared against an anti-H-2Kb mAb detected a public idiotype expressed on a majority of anti-H-2Kb alloantibodies. In contrast, no reactivity could be detected between these anti-idiotypes and a series of cytotoxic T cell clones of a similar specificity. The results suggested that either the recognition structures of T cells and B cells are substantially different or the allo-determinants recognized by these cells are not the same. Current efforts have been devoted to examining the nature of the allo-determinants recognized by cloned T cell populations as compared to those determinants recognized by alloantiboeis. In addition, anti-receptor antibodies specific for the CTL clones have been produced and attempts are being made generate mAbs that recognized the T cell receptor.